Function createModalProcessor

Create the modal barrier processor (highest pipeline priority).

The highest-z-index modal layer receives the event. Inside the modal, all active elements are offered the event (broadcast). The modal still consumes every unhandled event by default — the modal takes over the keyboard while open, so nothing beneath it ever sees unhandled keys; only keys registered via allowModal pass through to lower stages.

useModalMissListener callbacks are invoked for consumed keys that nothing on the modal layer handled, so the modal can react to unknown keys (e.g. a "key not bound" hint).